特殊形貌SAPO-44分子筛催化甲醇转化制低碳烯烃性能研究

Study on catalytic performance of SAPO-44 molecular sieve with special morphology for methanol conversion to light olefins

SAPO-44分子筛具有与SAPO-34分子筛同样的CHA拓扑结构,在甲醇转化制低碳烯烃(MTO)反应中具有优良的催化性能,但传统SAPO-44分子筛单一的微孔孔道结构限制了反应物和产物分子的扩散,分子筛的传质扩散性能有待进一步提高。通过水热法在合成体系中添加氯化铵制备了特殊形貌的SAPO-44分子筛,采用XRD、SEM、N_(2)吸/脱附和NH_(3)-TPD研究了合成体系中氯化铵的添加量对所合成SAPO-44分子筛的晶相结构、形貌、孔结构性质以及酸性质等物理化学性质的影响,并考察了所合成的SAPO-44分子筛在MTO反应中的催化性能。结果表明,氯化铵的添加降低了分子筛晶化体系的pH值;未添加氯化铵合成的SAPO-44分子筛的粒径为14.79μm,形貌为传统立方体,添加氯化铵后合成的SAPO-44分子筛呈新型特殊形貌结构,并且粒径减小。当n(NH_(4)Cl):n(Al_(2)O_(3))=0.25时合成的SAPO-44分子筛粒径为7.58μm,呈飞碟形结构;当n(NH_(4)Cl):n(Al_(2)O_(3))=0.50时合成的SAPO-44分子筛粒径为9.41μm,呈类似球形结构。特殊的形貌堆积使分子筛形成了介孔,构筑了微介孔结构,比表面积和孔容增大,酸强度和酸量增加。在MTO反应中,添加氯化铵合成的特殊形貌的SAPO-44分子筛生成的微介孔结构和粒径的减小有利于增强分子扩散效率,使反应稳定性提高1.4倍以上,同时,酸性的增强构成了更多的催化活性中心,以及分子筛的微介孔结构和小尺寸晶粒的共同作用使低碳烯烃(乙烯+丙烯)选择性提高3.00%以上。

SAPO-44 molecular sieve has the same CHA topological structure as SAPO-34 and has excellent catalytic performance in methanol conversion to light olefins(MTO)reaction.However,the single microporous pore structure of traditional SAPO-44 limits the molecular diffusion of reactants and products,and the mass transfer and diffusion performance of the molecular sieve needs to be further improved.SAPO-44 molecular sieve with special morphology was synthesized by adding ammonium chloride to the crystallization system by the hydrothermal method.XRD,SEM,N_(2) adsorption/desorption and NH_(3)-TPD were used to study the effects of the amount of ammonium chloride added to the synthesis system on the crystal phase structure,morphology,pore structure and acid properties and other physical and chemical properties of SAPO-44 molecular sieve.The catalytic performance of SAPO-44 molecular sieve in MTO reaction was also investigated.The results show that pH value of molecular sieve crystallization system is reduced by adding ammonium chloride.The particle size of SAPO-44 molecular sieve synthesized without ammonium chloride is 14.79μm,and the shape is traditional cube.The SAPO-44 molecular sieve synthesized adding ammonium chloride shows a new special shape morphology structure,and the particle size is reduced.When n(NH_(4)Cl):n(Al_(2)O_(3))=0.25,SAPO-44 molecular sieve has a particle size of 7.58μm and a flying saucershaped structure.When n(NH_(4)Cl):n(Al_(2)O_(3))=0.50,the size of SAPO-44 molecular sieve is 9.41μm,which shows similar spherical structure.The novel morphology is deposited in the molecular sieve to form mesoporous,constructs the micro-mesoporous structure,the specific surface area and pore volume are both increase,and the acid strength and acid content are also increase.In the MTO reaction,the reduction of micro-mesoporous structure and particle size generated by SAPO-44 molecular sieve with special morphology synthesized by adding ammonium chloride is conducive to enhancing the molecular diffusion efficiency and increasing the reaction stability by more than 1.4 times.Meanwhile,the enhancement of acidity forms more catalytic active centers.Moreover,the joint action of micro-mesoporous structure and small size particle can increase the selectivity of light olefins(ethylene+propylene)by more than 3.00%.